v3.25.2
Severance Indemnities and Pension Plans (Components of Net Periodic Cost of Pension Benefits and Other Benefits) (Detail) - JPY (¥)
¥ in Millions
 12 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Mar. 31, 2023
Defined Benefit Plan Disclosure [Line Items]      
Net periodic benefit cost (income) ¥ (63,046) ¥ (26,771) ¥ 45,710
Domestic, Japan | Pension benefits and SIPs      
Defined Benefit Plan Disclosure [Line Items]      
Service cost—benefits earned during the fiscal year 31,728 34,653 41,026
Interest cost on projected benefit obligation 23,954 20,547 14,356
Expected return on plan assets (95,929) (83,130) (84,084)
Amortization of net actuarial loss (gain) (16,821) (591) 1,446
Amortization of prior service cost (282) (1,974) (1,396)
Loss (gain) on settlements and curtailment (13,754) (13,659) (4,860)
Other (203) (194) 0
Net periodic benefit cost (income) (71,307) (44,348) (33,512)
Foreign offices and subsidiaries | Pension benefits      
Defined Benefit Plan Disclosure [Line Items]      
Service cost—benefits earned during the fiscal year 14,759 13,231 16,793
Interest cost on projected benefit obligation 8,850 8,461 20,634
Expected return on plan assets (12,443) (10,810) (44,590)
Amortization of net actuarial loss (gain) 2,112 1,362 7,534
Amortization of prior service cost (1,328) (945) (3,115)
Loss (gain) on settlements and curtailment 15 1,803 84,345
Other (2,513) 3,939 0
Net periodic benefit cost (income) 9,452 17,041 81,601
One-time write off 84,345    
Foreign offices and subsidiaries | Other benefits      
Defined Benefit Plan Disclosure [Line Items]      
Service cost—benefits earned during the fiscal year 52 52 97
Interest cost on projected benefit obligation 1,266 1,216 1,002
Expected return on plan assets (1,670) (1,503) (3,200)
Amortization of net actuarial loss (gain) (89) 1,191 155
Amortization of prior service cost (449) (426) (432)
Loss (gain) on settlements and curtailment 0 0 0
Other (301) 6 0
Net periodic benefit cost (income) ¥ (1,191) ¥ 536 ¥ (2,378)